1. Field of the Invention
The present invention relates to a four-wheel drive vehicle, and particularly to a four-wheel drive vehicle running both normally and with an object towed thereby.
2. Description of the Background Art
One of the purposes of the use of vehicles such as a SUV (sport utility vehicle) and a pickup truck is to run towing a trailer, a camping car or a cart loading a boat thereon which is coupled to the back of the vehicle.
Japanese Patent Laying-Open No. 2003-014100 discloses a technology aiming to increase durability of a transmission by engaging a lock-up clutch in a varied amount when it is determined that a vehicle tows a trailer and so on.
FIG. 6 is a diagram for illustrating a run while towing an object.
As shown in FIG. 6, when a vehicle 501 runs towing a trailer 502 coupled to the back of the vehicle, a higher driving torque than that in the normal run is transmitted to driving parts in the vehicle according to the towing operation.
Some of such vehicles employ a so-called part-time 4WD system which is capable of switching between a two-wheel drive mode having good gas mileage suitable for running in a normal mode on a dry road surface with high stability and a four-wheel drive mode suitable for running in the rain and snow or on a rough road.
In such a four-wheel drive vehicle capable of switching the driving state from the two-wheel drive mode to the four-wheel drive mode, driving parts of driving wheels driving only in the 4WD mode also need to be designed for towing. This may lead to an increase in size of peripheral parts of the driving wheels.
Furthermore, when a four-wheel drive vehicle with a center differential runs while towing an object in a four-wheel driving state, as shown in FIG. 6, owing to an applied force F1, a pressurizing force F3 which is applied to rear wheels becomes greater than a pressurizing force F2 applied to front wheels. This may cause a change in diameters between the front tires and the rear tires to lead a full-time differential of the center differential. As a result, durability of the differential may be impaired, and there is a need to solve these problems in towing by a four-wheel drive vehicle.